The Alveolar-arterial gradient (A-a gradient), is used to compare the causes of hypoxemia.

A – a gradient = PA_O2 – Pa_O2 Where:
PA_O2 = alveolar PO₂ (calculated from the alveolar gas equation)
Pa_O2 = arterial PO₂ (measured in arterial blood)

PA_O2 = PI_O2 – PA_CO2/R

Where:
PI_O2 = FI_O2 x (760 mmHg – 47mmHg) = inspired O₂
       – FI_O2 = 21% at sea level atmospheric pressure = 0.21
       – 760 mmHg = atmospheric pressure at sea level
       – 47 mmHg = H₂O pressure
PA_CO2 = arterial PCO₂ measured in plasma
R = respiratory exchange ratio or respiratory quotient = 0.8 under normal conditions

The normal A-a gradient is < 10 mmHg, but can range from 5-20 mmHg in a normal individual. An increased A-a gradient suggests a diffusion defect, V/Q (ventilation/perfusion) defect, or right-to-left shunt. ^[1]

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